Auxiliary switching circuit for a chopping converter

ABSTRACT

The invention relates to an auxiliary switching circuit ( 10 ) for a chopping converter comprising a first inductive element (L 0 ) for serial energy storage with a free-wheel diode (DL) and a switch (K), in addition to a second inductive element (L) for di/dt control when the switch is closed, the auxialiary switching circuit comprising a magnetic circuit ( 11 ) whereby a main winding thereof is formed at least partially by the first inductive element (L 0 ), also comprising means (L 1 , D 1 , L 2 , D 2 ) for discharging the second inductive element when the switch is opened or closed, and means (L 2 , D 2 ) for transferring the energy corresponding to the closure vis a vis said main winding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of power converters ofswitched-mode type. Such converters use an inductive element, associatedwith a power switch and with a free wheel diode, to perform a powerconversion and a correction of the power factor, generally based on aD.C. input voltage. Voltage step-down converters (BUCK), voltage step-upconverters (BOOST), and buck-boost converters are known.

The present invention more specifically relates to a circuit for helpingthe switching of the power switch of a switched-mode converter.

2. Discussion of the Related Art

FIG. 1 shows the simplified diagram of a conventional step-up converter1. Such a converter includes an inductance L0 in series with a freewheel diode DL between two positive input and output terminals 2 and 3of the converter, the cathode of diode DL being connected to terminal 3.A power switch K connects the midpoint 4 of this series connection to aterminal 5 of application of a negative or reference voltage (generally,the ground) common to the converter input and output. A D.C. supplyvoltage source 6 provides a voltage V_(E) across terminals 2 and 5. Onthe output side, a storage capacitor C0 generally connects terminals 3and 5 and provides a voltage V_(S) to a load Q. Load Q has been shown inFIG. 1 by dotted lines integrating capacitor C0, which may or not belongto the load. Switch K is controlled by a circuit 7 (CTRL), for example,in pulse-width modulation (PWM).

The operation of a step-up converter will now be described. When switchK is on, power is stored in inductance L0 and load Q is supplied by thepower stored in capacitor C0. When switch K is off, inductance L0 givesback the stored power to capacitor C0 via free wheel diode DL.

FIG. 2 shows the simplified electric diagram of a step-down converter1′. It shows the same components as in FIG. 1. However, here, switch Kis connected in series with inductance L0 between positive input andoutput terminals 2 and 3. Free wheel diode DL grounds the junction point4′ of switch K and inductance L0, its cathode being connected to point4′. Switch K may also be provided between the negative terminal ofsource 6 and the anode of diode DL.

The operating principle is the same. Power is stored in inductance L0during the on periods of switch K. During periods when switch K is off,this power is given back to capacitor C0, free wheel diode DL being usedto loop back the circuit.

A problem which arises with switched-mode converters, also calledhard-switching converters, in which the current and the voltage crosseach other upon each switching, is linked to the switch turning-on.

Indeed, upon each turning-on of switch K, free wheel diode DL mustblock. Now, at the blocking of a diode, especially of a PN junctiondiode, a recovered charge phenomenon occurs.

This phenomenon is illustrated by FIGS. 3A to 3C, which show, inrelation with the circuit of FIG. 1, an example of the shape of currentI_(DL) in the free wheel diode, of output voltage V_(S) and of currentI_(T) in switch K.

Switch K is initially assumed to be off. Accordingly, a current I_(Lf)flows through diode DL. This current corresponds to the power given backby inductance L0. The output voltage is at a level V0. As for switch K,the current I_(T) flowing therethrough is null.

It is assumed that at a time t1, control circuit 7 turns switch K on.During the switching, current I_(L) in the inductance, which correspondsto the sum of currents I_(DL) and I_(T) is a constant. Accordingly, thecurrent which, during the switching, increases in the switch, translatesas a decrease with an inverse slope of the current in diode DL.

At a time t2, the current in diode DL becomes zero and the current inthe switch reaches level I_(Lf). At this time starts the recoveredcharge phenomenon of diode DL. This known phenomenon translates as aninversion of the current through the diode to reach a level I_(RM)corresponding to the maximum recovery current of the diode. CurrentI_(RM) is reached at a time t3 from which the current through the diodetends towards zero again, reaching it at a time t4. Since the current ininductance L0 is, during the switching, substantially constant, thenegative current peak on the diode side translates as an overcurrent inswitch K, the maximum value of which corresponds to current I_(Lf) plusvalue I_(RM). On the side of voltage V_(S), the voltage decrease inpractice intervenes from time t3, that is, from the inversion of thecurrent slope in diode DL. In other words, the voltage across the diodeis zero between times t2 and t3 corresponding to the first recoveryphase ta. It can be considered that the diode then transiently conductsin reverse. Between times t3 and t4 (second recovery phase tb), voltageV_(S) decreases from V₀ to a zero voltage. The voltage provided tocapacitor C0 is here considered. Indeed, the presence of the capacitorin practice results in output voltage V_(S) remaining approximatelystable.

The slope between times t1 and t3 of the current decrease in diode DLdepends on the turn-on speed of the switch and thus on its di/dt at theturning-on. The higher this di/dt, which favors an abrupt switching, thehigher amplitude I_(RM) is for a PN-junction diode. However, the smallerdi/dt, the longer the recovery time at the blocking (trr=t4−t2).

The losses in a diode according to the di/dt value have a parabolicshape. There is an optimal point where the surface area of the currentshape between times t2 and t4 is minimum, which results in minimumlosses of recovered charges in the diode.

For switch K, the recovered charge phenomenon of the diode isparticularly disturbing. Indeed, for a step-up converter, the switchthen sees across its terminals, between times t2 and t3, output voltageV_(S). In the case of a step-down converter, the voltage seen by theswitch across its terminals corresponds to the voltage of generator 6.In all cases, it is the highest voltage between voltages V_(E) andV_(S).

High losses can then be observed in switch K. In FIGS. 3A to 3C, theloss periods have been symbolized by hatching on the various timingdiagrams.

In practice, the losses in switch K (generally, a power transistor) atits turning-on (times t1 to t4) form most of the switching losses of theconverter. In particular, the losses due to the actual blocking of thediode and the turn-off losses of the switch are negligible with respectto the losses generated at its turning-on.

A first solution to reduce this disadvantage consists of using diodeswith no recovered charges, for example, Schottky or SIC-type diodes.

A first disadvantage of this solution is that diodes with no recoveredcharges are often limited to a breakdown voltage of some hundred volts.This solution is thus not applicable to converters operating undervoltages of several hundreds of volts, which is in practice current inpower electronics. Several diodes in series must then be provided toincrease the breakdown voltage.

Another disadvantage of this solution is that, even if it decreaseslosses linked to recovered charges (times t2 to t4), the mostsignificant losses linked to the sole switch turning-on are not avoided.Referring to the example of FIGS. 3A to 3C, the use of a diode with norecovered charges results in an zero voltage V_(S) from time t2. Therethus remain the losses linked to the surface areas located between timest1 and t2.

Another disadvantage of diodes with no recovered charges is that theyare particularly expensive as compared to PN diodes. Presently, the costratio is greater than 20.

A second solution to attempt solving recovered charge problems is toprovide a circuit for helping the switching of the power switch of theconverter.

FIG. 4 shows a conventional example of such an aid circuit, applied to astep-up converter such as shown in FIG. 1. FIG. 4 shows all elements ofFIG. 1, to which is added a circuit 8 for helping the switching ofswitch K. This circuit is formed of an inductance L, associated inparallel with a resistor R and a diode D, between point 4 and switch K.The function of inductance L is to control the switch di/dt. Bydecreasing this di/dt value, amplitude I_(RM) is decreased.

A problem which arises is that resistor R must be provided to dissipatea reverse overvoltage in inductance L. Indeed, upon the turn-onswitching of switch K, the voltage across inductance L takes the valueof output voltage V_(S). The same losses occur at the transistorturning-off. These are resistive losses which are all the greater as thedi/dt value is high. In other conventional examples, dissipation elementR is replaced with a capacitor, a zener diode, etc.

Thus, this second solution has the same disadvantages as the use of adiode with no recovered charges.

A third known solution (not shown) consists of a circuit for helping theswitching using the transient switching resonance. Such a circuit uses,like the circuit of FIG. 4, an additional inductance. However, to avoidresistive loss problems, a second switch, the control of which isdesynchronized with respect to that of switch K, is used.

An example of a switching aid circuit of this type is described in paper“An overview of soft switching technics for PWM convertors” by G. Huaand F. Lee, published in EPE Journal, Vol. 3, March 1993.

Such a solution provides satisfactory results, but has a particularlycomplex and expensive implementation. In particular, a control systemdesynchronized from the used switches must be provided. Further, ascompared to the circuit of FIG. 4, it is necessary to have an additionalpower switch, two additional diodes and, above all, a high-voltagecapacitor.

The present invention aims at overcoming the disadvantages of knownswitching aid circuits.

SUMMARY OF THE INVENTION

The present invention more specifically aims at providing a switchingaid circuit which reduces losses due to the turning-on of a powerswitch.

The present invention also aims at providing a solution requiring noadditional switch in a lightly dissipative circuit.

The present invention also aims at providing a particularly simple andinexpensive solution.

The present invention also aims at providing a solution which iscompatible with the use of diodes with recovered charges (PN diodes).

The present invention also aims at preserving the control of the di/dtvalue upon turning-on of the power transistor.

To achieve these objects, the present invention provides a circuit forhelping the switching of a switched-mode converter, which includes afirst inductive power storage element in series with a free wheel diodeand a switch, and a second inductive element for controlling the di/dtvalue upon turning-on of the switch, including:

a magnetic circuit having a main winding formed, at least partially, bythe first inductive element;

means for discharging the second inductive element at the switchturning-off and turning-on; and

means for transferring the power corresponding to the turning-on to saidmain winding.

According to an embodiment of the present invention, said dischargemeans include:

a first circuit including a first switching diode; and

a second circuit including a first secondary winding of the magneticcircuit.

According to an embodiment of the present invention, said transfer meansinclude the first secondary winding of the magnetic circuit and a secondswitching diode.

According to an embodiment of the present invention, the seconddischarge circuit includes the second inductive element in series withthe first secondary winding, the second switching diode, and the switch.

According to an embodiment of the present invention, the switching aidcircuit further includes a second secondary winding of the magneticcircuit in series with the free wheel diode.

According to an embodiment of the present invention, the secondarywindings have a same number of turns.

According to an embodiment of the present invention, the number of turnsof the main winding is greater than the numbers of turns of thesecondary windings.

The present invention also provides a switched-mode converter of thetype including a first inductive power storage element in series with afree wheel diode and a storage element of capacitive type, and a secondinductive element for controlling the di/dt value upon turning-on of aswitch for cutting-off a supply voltage, including a switching aidcircuit.

According to an embodiment of the present invention, the converter is ofvoltage step-up type, the first inductive element forming the mainwinding of the magnetic circuit being in series with the secondinductive element and the switch between two terminals of application ofthe supply voltage.

According to an embodiment of the present invention, the converter is ofvoltage step-down type, the switch being in series with, among other,the second inductive element and the free wheel diode, between twoterminals of application of the supply voltage.

The foregoing objects, features and advantages of the present inventionwill be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, previously described, shows a conventional example of a voltagestep-up switched-mode converter;

FIG. 2, previously described, shows a conventional example of a voltagestep-down switched-mode converter;

FIGS. 3A, 3B, and 3C, previously described, illustrate in the form oftiming diagrams a problem posed by the circuits of FIGS. 1 and 2;

FIG. 4, previously described, shows another conventional example of avoltage step-up switched-mode converter;

FIG. 5 shows an embodiment of a switching aid circuit according to thepresent invention applied to a voltage step-up converter;

FIGS. 6A, 6B, 6C, 6D, 6E, 6F, and 6G illustrate, in the form of timingdiagrams, the operation of the circuit of FIG. 5;

FIGS. 7A, 7B, 7C, 7D, 7E, and 7F show the equivalent electric diagramsof the circuit of FIG. 5 at the different switching phases; and

FIG. 8 shows a first embodiment of a switching aid circuit according tothe present invention applied to a voltage step-down converter.

DETAILED DESCRIPTION

The same elements have been designated with the same references in thedifferent drawings. For clarity, only those components which arenecessary to the understanding of the present invention have been shownin the drawings and will be described hereafter. In particular, thestructure of the power switch control circuit has not been detailed andis not part of the present invention, its implementation being withinthe abilities of those skilled in the art based on the functionalindications given in the present description.

A feature of the present invention is to provide a magnetic circuit fororganizing the discharge of an inductance for controlling the di/dtvalue, especially, upon closing of the power switch of a switched-modeconverter.

Another feature of the present invention is to use this magnetic circuitto temporarily store the power generally lost upon switching of thepower switch and for storing this power in the converter to the benefitof the load.

Another feature of the present invention is to use the inductive elementof the circuit for correcting the power factor of the switched-modeconverter as an element of the magnetic circuit.

FIG. 5 shows the electric diagram of a first embodiment of a voltagestep-up switched-mode converter, equipped with a switching aid circuitaccording to the present invention.

As previously, power converter 10 includes a switch K controlled by acircuit (not shown), for example, a pulse-width modulation controlcircuit (PWM). A power storage inductance L0 is connected, by a firstterminal, to a positive terminal 2 of application of an input voltageV_(E) provided by a source 6 (for example, a D.C. source). Switch K isin series with an inductance L for controlling the di/dt value,connected to the second terminal 4 of inductance L0. The other terminalof switch K is connected to a reference terminal 5 (generally, theground). Conventionally still, a free wheel diode DL is placed betweenpoint 4 and a positive output terminal 3 of the converter. This positiveterminal is connected to a first electrode of a storage capacitor C0(which may belong to the load Q to be supplied) across which is presentoutput voltage V_(S). The other terminal of capacitor C0 is grounded andthe anode of diode DL is on the side of terminal 3.

According to the present invention, inductance L0 belongs to a magneticcircuit 11, of which it forms the main winding. Magnetic circuit 11includes two secondary windings L1 and L2 having respective numbers ofspirals or turns N1 and N2 smaller than number N0 of spirals ofinductance L0. A first winding L1 of magnetic circuit 11 is connected inseries with diode DL across terminals 3 and 4. In the example of FIG. 5,this inductance has been shown between point 4 and the anode of diodeDL. It may also be placed between the cathode of diode DL and terminal3, the anode of diode DL being then directly connected to point 4. Asecond winding L2 connects point 4 to terminal 5 by being associated inseries with a diode D2, the anode of diode D2 being directed towardsground 5. As for inductance L1 and diode D1, diode D2 may be, converselyto what has been shown, connected to point 4. Finally, a diode D1connects point 12 to terminal 3 between inductance L and switch K, thecathode of diode D1 being connected to point 12.

The function of winding L1 is, upon turning-off of switch K, to impose anegative voltage across inductance L, to enable it transfer the powerthat it contains to capacitor C0. Diode D1 is then forward biased.

Winding L2 has the function, upon turning-on of switch K, of imposing anegative voltage across inductance L, to transfer the power that itcontains into winding L2 of the magnetic circuit. This power isrecovered by winding L0 which gives it back to capacitor C0 at the nextswitch turning-off.

To respect these functionalities, the respective phase points of thewindings are chosen as follows. Assuming that the phase point of windingL0 is connected to terminal 2 as illustrated in FIG. 5, the phase pointof winding L1 must be on the side of point 4 and the phase point ofwinding L2 must be on the side of ground 5. Conversely, if the phasepoint of winding L0 is connected to point 4, the phase point of windingL1 must be on the side of terminal 3 and the phase point of winding L2must be on the side of point 4.

The operation of the switching aid circuit shown in FIG. 5 will bedescribed hereafter in relation with FIGS. 6A to 6G and 7A to 7F. FIGS.6A to 6G show, in the form of timing diagrams with no scaleconsideration, an example of a switching cycle of switch K. FIGS. 7A to7F show the equivalent electric diagrams of the circuit of FIG. 5 in thedifferent switching phases.

FIG. 6A shows voltage V_(DL) across free wheel diode DL. FIG. 6B showscurrent I_(DL) in diode DL. FIG. 6C shows voltage V_(K) across switch 4.FIG. 6D shows current I_(K) in the switch. FIG. 6E shows voltage V_(L)across di/dt-control inductance L. FIG. 6F shows current I_(D1) in diodeD1. FIG. 6G shows current I_(D2) in diode D2. The signs of the currentsand voltages shown in FIGS. 6A to 6G are taken in relation with thedirections indicated in FIG. 5. In FIGS. 7A to 7F, the current flowshave been symbolized by arrows.

It is assumed that before a time t10, switch K is off, the converterthen being in free wheel (phase A). During this free wheel period, acurrent I₀ assumed to be constant flows through diode DL, being givenback by inductances L0 and L1. During this phase A where switch K isoff, the equivalent diagram of the converter (FIG. 7A) only includesinductance L0 in series with inductance L1 and diode DL betweenterminals 2 and 3 to provide the power to the load and to capacitor C0.In FIG. 7A, forward-biased diode DL has been symbolized by ashort-circuit. Voltage V_(DL) across this diode is slightly positive andcorresponds to the voltage drop in the forward PN junction (on the orderof 0.7 V). Switch K sees across its terminals a voltage V₀ correspondingto voltage V_(S) plus voltage V_(DL) and decreased by the voltage dropin winding L1. Voltage V_(L) in inductance L is indeed zero during thisperiod, as will be seen hereafter in relation with the end of the timingdiagrams. Diodes D1 and D2 are blocked and the currents flowingtherethrough are accordingly null. Current I_(K) in off switch K is ofcourse null.

At time t10, the turning-on of switch K is controlled. This thus startsa turn-on beginning phase B, the equivalent diagram of which is shown inFIG. 7B. As compared to FIG. 7A, the only difference is that inductanceL in series with on switch K (short-circuit) is interposed between point4 and ground 5. The di/dt value upon turning-on of switch K essentiallydepends on inductance L. Indeed, this di/dt value depends on voltageV_(S), on voltage V_(E), on the mutual inductance of the magneticcircuit and on the off-load inductances L11 and L22 of the transformerformed by primary winding L0, and secondary windings L1 and L2. Due tothe chosen spiral ratio, value L11 is very large as compared to valueL22. The mutual inductance is moreover small as compared to value L11.As a result, slope (di/dt) is, as a first approximation, equal toV_(S)/L. Current I_(DL) through diode DL thus decreases with this slopeuntil a time t12. Since a PN junction is used, the diode exhibits arecovered charge area. Accordingly, current I_(DL) annuls at a time t11,intermediary between times t10 and t12. Time t11 corresponds to the timewhen the current in switch K reaches value I₀. Between times t10 andt12, diodes D1 and D2 remain blocked. Voltage V_(L) across inductance Lbecomes approximately equal to voltage V_(S).

At time t12, the current through diode DL reaches value I_(RM)corresponding to the maximum recovered charges. From time t12, thecharges recovered by diode DL decrease. Diode DL then behaves as acapacitor. The equivalent diagram of this operating phase C is shown inFIG. 7C where diode DL has been symbolized in the form of a capacitor.The rest of the elements are the same as in FIG. 7B. Since the number ofspirals of inductance L1 is small as compared to the number of spiralsof inductance L0, voltage V_(L1) thereacross is small. As a result, thecapacitance formed by diode DL charges negatively. This phenomenon isillustrated in FIG. 6B by a pursuit of the decrease of current I_(DL)until a time t13 in the form of a capacitor charge. The currentdecreases to a current I_(r) conditioned by inductance L2. Indeed,voltage V_(L), which decreases during this phase C, becomes negativeuntil diode D2 is turned on when voltage V_(L) becomes sufficientlynegative (time t13). As for diode DL, voltage V_(DL) reaches, at timet13, value −(V_(S)+V_(L1)+V_(L2)+V_(D2)). Voltage V_(L) reaches, at timet13, value −(V_(K)+V_(L2)+V_(D2)).

At time t13 when diode D2 turns on, current I_(DL) through diode DLabruptly stops and the corresponding current is injected back intoinductance L2. The excess current (I_(r)) gives the maximum amplitude ofthe current in inductance L2. This current depends on the numbers ofspirals N0 and N2 of inductances L0 and L2. From time t13, diode D2conducts (phase D). The equivalent diagram is illustrated in FIG. 7D.Since diode DL is blocked (non-conducting), capacitor C0 isdisconnected. The magnetic circuit is, during phase D, dissociated fromload Q. Diode D2 is then used as a free wheel element to transfer thepower stored by inductance L into the magnetic circuit via winding L2.The voltages across diode DL and inductance L remain unchanged.Similarly, switch K being on, the voltage thereacross is zero. Diode D1is blocked. When the current is entirely transferred into the magneticcircuit by inductance L2, the current therein goes to zero (time t14),which causes a natural blocking of diode D2, that is, with a smalldi/dt. Winding L2 enables decreasing of the current in switch K bytransferring the power to the magnetic circuit which will give it backthrough inductance L0. Between times t13 and t14, the current in switchK will decrease from level I₀+I_(r) to level I₀.

At time t14, the voltage across inductance L goes to zero, all the powerthat it contained having been transferred to the magnetic circuit. Thevoltage across diode DL slightly rises back while remaining negative andtakes a value −(V_(S)+V_(L1))+V_(L)+V_(K). It should be reminded thatvoltages V_(L) and V_(K) are negligible (considered as null) withrespect to voltages V_(S) and V_(L1).

Time t14 is the beginning of a phase E where the switch is on and wherethe switching is over. The equivalent diagram is shown in FIG. 7E. Itonly includes source 6, inductances L0 and L, and switch K. CurrentI_(K) is stable at level I₀, as well as voltage V_(DL), the free wheeldiode being blocked. The voltage across switch K of course is zero, aswell as the voltage across inductance L and the currents in diodes D1and D2. During phase E, inductance L0 is loaded through inductance L andswitch K.

At a time t15 when switch K is turned off, a negative voltage is imposedacross inductance L, due to the presence of winding L1. It should benoted that, in this case, it is not necessary to control the di/dt valueupon turning-off of the transistor (conventionally). The currentabruptly stops in switch K. The inversion of the voltage acrossinductance L1 causes the discharge, through diode D1, of the powerstored during phase E in inductance L. At time t15, current I_(D1) thusabruptly takes value I₀ and this current decreases to reach value zeroat a time t16. The decrease slope of current I_(D1) is a function of thevalue of inductance L and approximately corresponds to V_(L1)/L. Thecurrent through inductance L goes to zero at time t16 and all thecurrent accumulated in winding L0 then flows through winding L1 anddiode DL. The equivalent diagram of phase F is illustrated in FIG. 7F.It should be noted that diodes DL and D1 are on at the same time, butthe current through diode DL starts from zero at time t15.

Time t16 starts a new phase A where the switch is off.

An advantage of the present invention is that it enables recovering thelosses due to the turn-on switching of the power switch to inject themback into the load by means of the magnetic circuit. The reinjection ofthe current into the converter, during turn-on switching phase D of theswitch, enables decreasing the duty cycle. The controller (controlcircuit of switch K) generally automatically decreases this duty cycleby a regulation means which is not part of the present invention. Asignificant improvement of the converter efficiency is thus hereobtained.

Another advantage of the present invention is that the provided solutionis particularly simple. As compared to the conventional circuit of FIG.4, one power switch and, above all, a complex control circuit, arespared.

Another advantage of the present invention is that it requires nomodification of the power switch control circuit, provided that saidcircuit performs (which is generally the case) a regulation. Theimplementation of the present invention requires adding one magneticcircuit L0, L1, L2, which can be obtained by means of a singlethree-winding inductance. Such a magnetic circuit is considerably lessexpensive than the required complexity of the control circuit of FIG. 4and than a diode with no recovered charges. On this regard, it should benoted that the solution of a diode with no recovered charges does notenable recovering the losses in the switch.

FIG. 8 shows another embodiment of a switching aid circuit 10′ of thepresent invention, applied to a voltage step-down converter. The diagramof FIG. 8 should be compared to that of FIG. 2. As compared to thediagram of FIG. 2, inductance L is interposed between point 4′ andswitch K. Inductance L2 in series with diode D2 is connected betweenterminal 2 and point 4′, the anode of diode D2 being on the side ofterminal 2. Winding L1 is connected in series with diode DL betweenpoint 4′ and ground 5, the anode of diode DL being on the ground side.Finally, diode D1 connects to ground 5 point 12 between switch K andinductance L, the anode of diode D1 being grounded. In the example ofFIG. 8, the phase point of winding L0 is connected to point 4′.Accordingly, to fulfill the described functions of magnetic circuit 11′,the phase point of winding L1 is on the side of ground terminal 5 andthe phase point of winding L2 is on the side of terminal 2.

The operation of the switching aid circuit illustrated in FIG. 8 can bededuced from the discussion of FIGS. 5 to 7.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, the sizing of the different windingsof the magnetic circuit may be modified, provided to respect a windingL0 having a number of spirals much greater than windings L1 and L2.Preferably, the numbers of spirals of windings L1 and L2 are equal, andthe number of spirals of winding L0 is approximately 10 times greaterthan that of windings N1 and N2.

Further, adapting the present invention to a buck-boost converter iswithin the abilities of those skilled in the art based on theindications given hereabove.

Further, the present invention applies to any converter assembly,provided that it is a switched-mode converter. In particular, if in thecase of a step-down converter (FIG. 8), the switch has been shown with aterminal connected to the most positive voltage, there also existassemblies in which this switch has a grounded terminal. The presentinvention also applies to this type of assembly. It is sufficient toinvert the respective positions of series associations K–L and L1–DLwith respect to point 4′, to connect diode D1 by its cathode to terminal2, and to place series association L2–D2 in parallel on association K–L,the cathode of diode D2 remaining connected to node 4′. Inductance L0still is connected on the cathode side of free wheel diode DL in serieswith capacitor C0.

Finally, among the possible alternatives, inductance L0 may be dividedinto a (main) element of the magnetic circuit in series with a distinctinductance that does not belong to the magnetic circuit. The switchingspeeds of the diodes may also be adapted although, to obtain theadvantages of the present invention, these diodes need not be fast.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be within and scope ofthe invention. Accordingly, the foregoing description is by way ofexample only and is not as limiting. The invention is limited only asdefined in the following claims and the equivalents thereto.

1. A circuit for helping the switching of a switched-mode converter,which includes a first inductive power storage element in series with afree wheel diode and a switch, and a second inductive element forcontrolling the di/dt value upon turning-on of the switch, characterizedin that it includes: a magnetic circuit having a main winding formed, atleast partially, by the first inductive element; discharge means fordischarging the second inductive element at the switch turning-off andturning-on; and transfer means for transferring the power correspondingto the turning-on to said main winding.
 2. The circuit of claim 1,wherein said discharge means include: a first discharge circuitincluding a first switching diode; and a second discharge circuitincluding a first secondary winding of the magnetic circuit.
 3. Thecircuit of claim 2, wherein said transfer means include the firstsecondary winding of the magnetic circuit and a second switching diode.4. The circuit of claim 3 wherein the second discharge circuit includesthe second inductive element in series with the first secondary winding,the second switching diode, and the switch.
 5. The circuit of claim 2wherein it further includes a second secondary winding of the magneticcircuit in series with the free wheel diode.
 6. The circuit of claim 5,wherein the secondary windings have a same number of turns.
 7. Thecircuit of claim 5 wherein the number of turns of the main winding isgreater than the numbers of turns of the secondary windings.
 8. Aswitched-mode converter of the type including a first inductive powerstorage element (L0) in series with a free wheel diode and a storageelement of capacitive type, and a second inductive element forcontrolling a di/dt value upon turning-on of a switch that cuts-off asupply voltage, including the switching aid circuit (10, 10′) ofclaim
 1. 9. The converter of claim 8, of voltage step-up type, whereinthe first inductive element forming the main winding of the magneticcircuit is in series with the second inductive element and the switchbetween two terminals of application of the supply voltage.
 10. Theconverter of claim 8, of voltage step-down type, wherein the switch isin series with, among other, the second inductive element and the freewheel diode, between two terminals of application of the supply voltage.